Method and device for the continuous cooking of pulp
The continuous digester system has an inlet defined therein for the feed of a chips suspension and an outlet for the output of a cooked suspension of pulp. The suspension of chips is fed in to the inlet through a line at the beginning of the cook, where the chips suspension has a volume of starting cooking fluid that establishes a fluid/wood ratio that is greater than 3.5. A final cooking fluid is present during the cook for the major part of the cook and is withdrawn through a withdrawal strainer only during the final 15 minutes of the cook. The final cooking fluid ensures a fluid/wood ratio that is greater than 3.5 in association with the withdrawal. The withdrawn final cooking fluid during the final 15 minutes of the cook consists of spent black liquor that maintains a residual alkali level that lies under 15 g/l.
1. An arrangement for continuous cooking of chemical cellulose pulp in a continuous digester system, for regulation of an H factor of hemicellulose dissolved in a cooking liquor, comprising:
a continuous digester system having an inlet defined therein for a continuous feed of a chips suspension, the continuous digester system having an outlet defined therein for a continuous output of a cooked suspension of pulp,
a feed line in fluid communication with the inlet, the chips suspension having, at an upper part of the digester system, a volume of a starting cooking fluid that is sufficient to establish a fluid/wood ratio that is greater than 3.5,
the starting cooking fluid being constituted by one or several of a fluid condensate, fresh cooking fluid, chips moisture, washing filtrate and spent black liquor, the spent black liquor having been present during at least 75% of a total duration of the cook,
a volume of final cooking fluid being a partial volume of the starting cooking fluid, the final cooking fluid being present in the cook for a major part of the cook prior to being withdrawn through a withdrawal section arranged at a height (h) extending from an upper edge of the withdrawal section to a bottom flange at the outlet of the digester, the height (h) being less than twice a diameter of the digester, the withdrawal section constituting a lowermost withdrawal section of the digester, means for ensuring a fluid/wood ratio of the final cooking fluid at the withdrawal section of at least 3.5,
the withdrawal section having a size of a withdrawal section area in square meters being a factor k multiplied by a current production in tons of pulp per day wherein the factor k≧0.06,
the withdrawal section area of the withdrawal section constituting more than 70% of a total area of withdrawal sections of the digester, and
a diameter of the digester being greater than 5 meters.
2. The arrangement according to claim 1 wherein a recovery processing unit (REC) is in fluid communication with the withdrawal section via a withdrawal line and has means for receiving more than 80% of any withdrawal from the withdrawal section.
3. The arrangement according to claim 2 wherein the recovery processing unit has means for receiving more than 90% of the withdrawal from the withdrawal section.
4. The arrangement according to claim 3 wherein the recovery processing unit has means for receiving more than 95% of the withdrawal from the withdrawal section.
5. The arrangement according to claim 1 wherein a partial volume of a withdrawal in a withdrawal line is in fluid communication with the starting cooking fluid in the feed line via a return line extending between the feed line and the withdrawal line.
6. The arrangement according to claim 1 wherein the factor (k)) is within an interval 0.08-0.12.